Abstract : Pyrophyllite which is used in ultra high pressure apparatus for supporting anvils, or gaskets, etc., often behaves differently. It is shown here that the main reason for this difference is due to a difference in density which in turn is apparently due to a difference in porosity. If the various pyrophyllite samples are precompressed to a sufficiently high pressure prior to being machined for final use the different behavior nearly disappears. Moreover, the resulting densities of the precompressed specimens are nearly the same. Relations between the work hardening of the anvils and the density of the pyrophyllite is discussed. (Author)